The parliament announced it was submitting a planning application for the structure, intended to increase security by ensuring visitors are screened before entering the main building.
It also released an artist’s impression of how the extension would fit with the existing design of the parliament.
In a letter to MSPs, presiding officer Tricia Marwick said the structure would only be built if the cost and timescale were “acceptable”.
But a report to the cross-party Scottish Parliamentary Corporate Body (SPCB) warned members that according to legal advice they could be open to prosecution if they failed to carry out measures considered “reasonably practicable” and someone was killed or injured.
The parliament has already spent around £2m on other extra security measures including turnstiles at key entrances, bollards along the Royal Mile and concrete benches at the front of the building.
The Evening News revealed two years ago how security bosses had called for an “external screening facility” because of fears of a suicide bomber.
The latest report to the SPCB said an external security facility would greatly improve the parliament’s “duty of care” to all who access the building.
“It would be linked to the building via a walkway which would provide the fundamental means of isolating threats either inside the facility or between the facility and the existing building.
“The facility would incorporate current blast technology and materials and, crucially, would be designed to minimise damage to the existing building and to the immediate surrounding environment which are the areas significantly occupied by visitors, staff and members during the week.”
But Green MSP Patrick Harvey said the extension was costly and pointless.
He said: “The Holyrood building opened later and cost more not least because it was built to the highest post-9/11 security standards.
“There are no active threats to Parliament that we have ever been made aware of, and we have never been the target of any serious security incident.
“Parliament should not make people who do come here feel like they are being treated as a threat or intrusion.
“This proposal, like the absurd bollards down the Royal Mile . . . is part of a long-standing ‘war on terror’ myth designed to separate politicians from the people they represent and to build a sense of fear and exclusion around public spaces.”
Independent Lothians MSP Margo MacDonald said MSPs should be shown the risk assessments for building or not building the extension.